Cross-interaction constants, ${ ho}^e_{XY}$, ${ ho}_{YZ}$ and ${ ho}_{XZ}$ are defined using observed rate constant, k_N=(k_1/k_{-1})k_2=Kk_2$, for the stepwise carbonyl addition reactions involving the rate-limiting breakdown of a tetrahedral intermediate $(T^{pm})$. Abundant experimental evidence in the literature enables us to determine signs for the three constants for such mechanism, ${ ho}^e_{XY}$>0, ${ ho}_{YZ}$<0 and ${ ho}_{XZ}$0. These are in contrast to those for the concerted $S_N2$ mechanism, ${ ho}_{XY}$<0, ${ ho}_{YZ}$>0 and ${ ho}_{XZ}$, and provide useful mechanistic criteria. In the light of these criteria, mechanisms of some nucleophilic reactions of carbonyl compounds are re-examined.